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php

Class cast

php.internals

David Rodrigues

6 years ago

Currently PHP supports generic castings like (string), (int), ... maybe is
time to allow class castings like (ToClass) $fromObject?

I think that it could be useful to converts to another kind of structure,
or even to reduce object type.

For instance:

---

1. Converting from A to B:

$a = 123;
$b = (Number) $a; // $b is now instanceof Number

A instanceof Number will created based on $a value.

---

2. Reduce object type (I don't know the technical term):

class A {}
class B extends A {}

$b = new B;
$a = (A) $b;

$a still is $b, but with "limited" access to A methods and properties.

---

To make possible custom some kind of castings, I suggests a new magic
method __cast($value).

class Number {
...
function __cast($value) {
if (is_int($value) || is_float($value)) return new static($value);
throw new TypeError();
}
}

Just to talk.


Atenciosamente,
David Rodrigues

Benjamin Morel

6 years ago

>
> Currently PHP supports generic castings like (string), (int), ... maybe is
> time to allow class castings like (ToClass) $fromObject?

I've proposed something similar a year ago:
https://externals.io/message/105332

My intention wasn't to create an object from a scalar, nor was it to limit
to a class' methods & properties (hardly possible in PHP), but to ensure
that the value is an instance of the given class/interface, or throw an
exception.
My proposal wasn't so well received, though.

— Ben

Stas Malyshev

6 years ago

Hi!

> 1. Converting from A to B:
>
> $a = 123;
> $b = (Number) $a; // $b is now instanceof Number
>
> A instanceof Number will created based on $a value.

What's wrong with new Number(123)?

> 2. Reduce object type (I don't know the technical term):
>
> class A {}
> class B extends A {}
>
> $b = new B;
> $a = (A) $b;
>
> $a still is $b, but with "limited" access to A methods and properties.

Not sure why would you want to do this? What's the use case?

--
Stas Malyshev
smalyshev@gmail.com

David Rodrigues

6 years ago

Hello!

> What's wrong with new Number(123)?

Actually it is a simplest case, where only a single information was
considered. But you can expand it to something more complex, where
currently you will need create a static method to copy the data, which is
not a normalized way like cast could do.

> Not sure why would you want to do this? What's the use case?

You could mix the previous answer when arguments to a method that expects a
Number, for instance: X::test((Number) $x).

Or you can increase the type of information when possible. For instance:

function requiresB(B $b);

But you have only A $a.

You can create:

function __cast($value) {
if ($value instanceof A) return B($value, 123, M_PI);
}

Then use:

requiresB((B) $a);

Atenciosamente,
David Rodrigues

Em qua., 25 de mar. de 2020 às 20:06, Stanislav Malyshev <
smalyshev@gmail.com> escreveu:

Stas Malyshev

6 years ago

Hi!

> Actually it is a simplest case, where only a single information was
> considered. But you can expand it to something more complex, where
> currently you will need create a static method to copy the data, which
> is not a normalized way like cast could do.

You can always use anonymous functions if callable is needed.

>
> Or you can increase the type of information when possible. For instance:
>
> function requiresB(B $b);
>
> But you have only A $a.
>
> You can create:
>
> function __cast($value) {
>     if ($value instanceof A) return B($value, 123, M_PI);
> }

That's not a cast. That's creating new object from another object.
Things like this should be an explicit call. Otherwise the meaning of
this code is unclear - it pretends that you just change the type, but in
fact it's a completely new object which can be entirely unrelated to the
old one. Things like that should be explicit.

--
Stas Malyshev
smalyshev@gmail.com